[Dovecot] POP3 flag updates

Why does Dovecot add \Seen flag for mails that have been RETRed? Does
anyone really want to use this feature? I can't remember why I
implemented it originally, perhaps it was just "because other servers
do that too". I'm thinking about at least changing
pop3_no_flag_updates=yes to be default.

There's also of course pop3_enable_last setting to enable LAST command
which was removed from POP3 specs. That requires knowing the last
RETRed mail, so I guess I should add a check that it can't be enabled
without pop3_no_flag_updates=no.